(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of creating Shallow Trench Isolation in regions of sub-micron device feature sizes.
(2) Description of the Prior Art
In fabricating semiconductor devices, typically in the surface of semiconductor substrates, many different disciplines of the art are applied. These disciplines are directed at creating particular features within the device. The combination of the device features form the finished device that has been designed and fabricated to perform a particular function and as such is referred to as an active device. Areas within the surface of the substrate that are dedicated to the formation of separate functions must electrically be isolated from each other, this has given rise to a range of methods that implement electrical insulation between device features and devices within a semiconductor die.
The continuing trend in the semiconductor industry is to form semiconductor devices on silicon substrates that have increasingly higher device densities and smaller device feature sizes. This continued device shrinkage and increased device density brings with it new problems, in particular one such problem relates to the necessity of providing an efficient and reliable process to separate active devices that function on the current miniaturized scale. For the Very Large Scale Integration (VLSI) and Ultra Large Scale Integration (ULSI) technologies, minimum device feature size of 0.25 um. is being approached. In the fabrication of such integrated circuits one of the most favorite dielectric isolation schemes by which one region of the semiconductor substrate is electrically isolated from another region is the Shallow Trench Isolation (STI).
One other method that has previously been used to create isolatio regions is the so-called xe2x80x9cLocal Oxidation of Siliconxe2x80x9d. (LOCOS) process. The LOCOS process is frequently used to form CMOS gate structures. For this LOCOS process, a temporary patterned nitride layer is used as a protection or resistant area to cover the future active areas during the subsequent field oxidation process. The LOCOS process can further briefly be described as follows: a pad oxide is formed on the surface of a silicon substrate, a layer of silicon f(Si3N4) is deposited over the layer of pad oxide. The pad oxide is thin thermal oxide that allows better adhesion between the nitride and silicon and acts as a stress relaxation layer during field oxidation. The;nitride and oxide layers are patterned and etched thereby creating openings exposing,those portions of the silicon substrate where the local oxidation is to be formed. A boron channel stop layer is ion implanted into the isolation regions (of the CMOS device). The field oxide is grown within the openings and the nitride and the pad oxide layers are removed. This completes the local oxidation. A disadvantage of the LOCOS process is that the process requires long oxidation times (thermal budget) and that lateral oxidation under the barrier mask (xe2x80x9cbird""s beak encroachmentxe2x80x9d) limits the minimum spacing between adjacent active device areas to about the 1 um range. This prevents further increases in device packaging density using the LOCOS process.
One method of circumventing the LOCOS limitations and to further reduce the field oxide (FOX) minimum feature size is to use shallow trench isolation (STI).
In using the STI approach for the VLSI technology, deep trenches are typically made in the substrate by reactive ion etching. The trenches are typically about 5-6 um. deep, about 2-3 um. wide and spaced about 2.5.-3.5 um. apart from another trench. The ULSI technology requires trenches that are deeper and spaced closer together posing new problems of field turn-on, punchthrough, gap-fill within the trenches and others. STI""s can be made using, for instance, Buried Oxide (BOX) isolation for the shallow trenches. The method involves filling the trenches with a chemical vapor deposition (CVD) silicon oxide (SiO2) and then etching back or mechanically/chemically polishing the SiO2 to yield a planar surface. The shallow trenches etched for the BOX process are anisotropically plasma etched into the silicon substrate and are, for ULSI applications, typically between 0.5 and 0.8 micrometer (um.) deep. STI are formed around the active device to a depth between 4000 and 20000 Angstrom.
Following the forming of the trenches they are filled with a suitable dielectric material such as oxide, polysilicon or an organic polymeric material, for example polyimide. While the dielectric-filled trench isolation can provide effective dielectric isolation between devices, the fundamental disadvantage of this scheme is that the resulting structure tends to be non-planar. This lack of planarity is mainly due to the difference in the amount of fill that is required to fill a multiplicity of closely spaced trenches and the dielectric that is deposited on the surface of the substrate. This effect is further aggravated by the steps of bake and cure that are applied to the deposited dielectric in order to cure the dielectric and to evaporate the solvents from the dielectric. Further problems can be caused in this respect by the fact that in many chip designs there can be a significant difference in device density across the chip. In the design of memory chips for instance, the memory functions of the chip can consist of 10.000 or more memory elements. These memory elements are surrounded by their supporting logic functions which tend to have considerably lower density of active elements thereby further aggravating the problems of even distribution of the deposited dielectric across the surface of the chip and of obtaining good planarization for the entire surface of the chip. It is clear that poor planarity across the surface of the trenches leads to further problems in creating interconnect patterns and in depositing overlying layers of insulation and metalization. These overlying layers of metalization must be patterned and etched, a typically photolithographic process that requires constant and low depth of focus. Where this depth of focus is not as required, wire patterns of poor quality are created resulting a serious yield detractors and concerns of device reliability.
Another problem associated with the formation of STI regions is that if the silicon oxide is etched or polished to the surface of the silicon substrate, dishing occurs in the surface of the silicon oxide resulting in a concave surface of the STI regions. This results in recesses in the field oxide at the edge of the device areas. Later, when the gate electrodes are made for the FET""s, the gate electrodes extend over the device area edge, causing an undesirable lower and variable threshold voltage when the devices are completed. It is therefore desirable to make isolation areas that extend higher than the substrate surface to avoid this problem while reducing manufacturing costs.
It must further be observed that recent requirements for the creation of holes within deep layers of either conducting or other materials have resulted in creating openings that have aspect ratios in excess of 3. It is beyond the capability of the existing techniques to fill gaps of this aspect ratio with High Density Plasma-oxide (HDP-oxide). This lack of adequate filling of gaps also occurs for holes that have a reentrant spacer profile. A reentrant spacer profile is a profile where the walls of the openings are not vertical but are sloped; this sloping of the walls makes complete penetration of the HPD-oxide into the hole difficult and, under certain conditions, incomplete.
As a consequence of incomplete deposition of HDP-oxide into high aspect ratio holes, keyholes or deposition irregularities will be formed. These keyholes or deposition irregularities are characterized by non-homogeneous deposition that form in the deposited HDP-oxide.
The indicated condition for the formation of a keyhole can also occur where a high aspect ratio through-hole is formed by RIE and where the formation position of the through-hole may deviate from the correct position due to mask misalignment or a process variation. The created through-hole can in this case exhibit a profile that inhibits complete and uniform deposition of HDP-oxide.
U.S. Pat. No. 5,783,476 (Arnold) shows a method comprising: form STI using 1) an O2 I/I into the trench, see FIG. 4, see Col. 3; 2) anneal to form SiO2, see Col. 3, line 47 and 3) trench fill with HPD oxide, see Col. 4, line 18. Arnold is close to the invention, see broad claim 1.
U.S. Pat. No. 5,807,784 (Kim) shows method comprising: form STI using 1) a O2 I/I into the trench; 2) anneal to form SiO2 and 3) trench fill with oxide. See claim 1, this is close.
U.S. Pat. No. 5,393,693 (Ko et al.) shows an isolation process using an O2 implant.
U.S. Pat. No. 5,670,388 (Machesney et al.) shows a SIMOX implant under a FET.
A principle objective of the invention is to provide a method of creating a high aspect ratio STI region while avoiding problems of gap fill and the occurrence of voids within the gap.
Another objective of the invention is to reduce the spacing between adjacent STI regions.
Yet another objective of the invention is to improve STI punchthrough immunity.
Yet another objective of the invention is to improve STI field turn-on/turn-off characteristics.
Yet another objective of the invention is to increase the STI junction breakdown voltage value.
Yet another objective of the invention is to avoid trench bottom corner damage in the STI opening.
In accordance with the objectives of the invention a new method is provided to create STI regions.
Under the first embodiment of the invention, a layer of pad oxide is deposited over the surface of the substrate; a layer of silicon nitride is deposited over the layer of pad oxide. The layers of pad oxide and silicon nitride are patterned and etched over the region where the STI is to be created, the (silicon) substrate is subjected to a tilt angle nitrogen implant that is self aligned with the opening that has been etched through the layers of silicon nitride and pad oxide. A deep shallow trench is now etched into the substrate; the patterned layer of photoresist is left in place. A tilt oxygen implant of moderate intensity is performed in the created opening, the photoresist is removed. An anneal is performed on the implanted oxygen. A liner oxide is grown within the opening, High Density Plasma (HDP) oxide is deposited inside the opening and on the surface of the remaining silicon nitride. CMP is performed to the surface of the HDP oxide down to the surface of the silicon nitride. After removing the silicon nitride and pad oxide, the STI region is completed under the first embodiment of the invention.
Under the second embodiment of the invention, a layer of pad oxide is deposited over the surface of the substrate; a layer of silicon nitride is deposited over the layer of pad oxide. The layers of pad oxide and silicon nitride are patterned and etched over the region where the STI is to be created, the (silicon) substrate is subjected to a tilt angle nitrogen implant that is self aligned with the opening that has been etched through the layers of silicon nitride and pad oxide. A deep shallow trench is now etched into the substrate; the patterned layer of photoresist is left in place. An oxygen implant of moderate intensity is performed in the created opening, the photoresist is removed. An anneal is performed on the implanted oxygen. A liner oxide is grown within the opening; the bottom of the created opening is now subjected to an oxidation process creating a layer of LOCOS on the bottom of the STI region. High Density Plasma (HDP) oxide is deposited inside the opening and the top surface of the remaining silicon oxide and silicon nitride. CMP is performed to the surface of the HDP oxide down to the surface of the silicon nitride. After the removal of the silicon nitride and the pad oxide has been completed, the STI region is completed under the second embodiment of the invention.
Under the third embodiment of the invention, a layer of pad oxide is deposited over the surface of the substrate; a layer of silicon nitride is deposited over the layer of pad oxide. The layers of pad oxide and silicon nitride are patterned and etched over the region where the STI is to be created, the (silicon) substrate is subjected to a tilt angle nitrogen implant that is self aligned with the opening that has been etched through the layers of silicon nitride and pad oxide. A deep shallow trench is now etched into the substrate; the patterned layer of photoresist is left in place. An oxygen implant of moderate intensity is performed in the created opening, the photoresist is removed. An anneal is performed on the implanted oxygen, the bottom of the opening is removed further increasing the depth of the opening while leaving spacers on the sidewalls of the opening. A liner oxide is grown within the opening followed by blanket High-Density Plasma (HDP) deposition of oxide. CMP is performed to the surface of the HDP oxide down to the surface of the pad oxide, which completes the formation of the STI region under the third embodiment of the invention.